Interlayer Bonding and the Lattice Vibrations ofβ-GaSe

The lattice vibrations of the layer compound $\beta$-GaSe have been investigated by means of infrared and Raman experiments. Reflectivity measurements for $\overrightarrow{\mathrm{E}}\perp c$ over the range 175-4100 ${\mathrm{cm}}^{-1\mathrm{}}$ have shown one infrared-active mode at 213.9 ${\mathrm{cm}}^{-1\mathrm{}}$. Six Raman lines have been observed at 19.1, 60.1, 134.6, 213.1, 249, and 307.8 ${\mathrm{cm}}^{-1\mathrm{}}$. A group-theoretical analysis of the lattice vibrations, which shows the origin of the conjugate modes in layer compounds, is presented. Electrostatic as well as van der Waals coupling between the layers is indicated by the large localized effective charge obtained from the infrared measurements.